This invention relates to methods for preparing processable high molecular weight polyaniline, to the processable polyaniline itself, to methods of preparing conductive fibers and films therefrom, and to such fibers and films and to conductive fiber composites.
Because of its excellent stability and the relatively high levels of electrical conductivity of certain forms of the material, much attention has recently been directed to polyaniline. The base form of the polymer in the emeraldine oxidation state (y=0.5), hereinafter referred to as "emeraldine base" or "polyaniline base" can be represented by the formula ##STR1## which contains equal numbers of alternating reduced, ##STR2## and oxidized, ##STR3## repeat units. The polymer can be protonated (or "doped") by dilute aqueous protonic acids such as HCl to produce the corresponding salt (A=anion): ##STR4## The polymer so obtained exhibits conductivities of about 1-5 S/cm, as compressed pellets of this powder, when approximately half of its nitrogen atoms are protonated as shown above.
For many useful applications of the conducting polyaniline polymers, it is necessary that they in some way be processable, i.e., capable of being converted into shaped articles such as fibers, films and fiber composites. It has been disclosed that free-standing films of emeraldine acetate (conductivity of about 0.5-2 S/cm) can be cast from solutions of emeraldine base in 80% aqueous acetic acid. M. Angelopoulos, A. Ray, A. G. MacDiarmid and A. J. Synth. Met., 21, 21-30 (1987). M. Angelopoulos, G. E. Asturias, S. P Ermer, A. Ray, E. M. Scherr, A. G. MacDiarmid, M. Akhtar, Z. Kiss and A. J. Epstein, Mol. Cryst. Liq. Cryst., 1988, Vol. 160, pp. 151-163, disclose that the best solvent for producing high quality free-standing films of emeraldine base is 1-methyl-2-pyrrolidinone (NMP), and that the mechanical properties of films cast from NMP are greatly superior to those of films cast from 80% acetic acid as previously described.
It has recently been disclosed that polyaniline may be successfully processed into monofilaments from solutions in concentrated sulfuric acid. A. Andreatta, Y. Cao, J. C. Chiang, A. J. Heeger and P. Smith, "Electrically Conductive Fibers of Polyaniline Spun from Solutions in Concentrated Sulfuric Acid," Synth Met., 26 (1988) pp 383-389. The polyaniline derived from sulfuric acid solutions (up to 20% w/w) is disclosed as displaying sharp X-ray reflections, indicative of a significant degree of crystallinity, and the electrical conductivity of as-spun, washed and dried fibers was in the range from 20-60 S/cm. Andreatta et. al. estimate that the molecular weight of the polyaniline used to prepare their fibers is between 12,000 and 40,000.
It has been disclosed to remove impurities and/or low molecular weight fractions from emeraldine base by extraction with solvents such as THF, DMSO, pyridine, CH.sub.3 CN, 80% acetic acid and 60% formic acid. A. G. MacDiarmid, J. C. Chiang, A. F. Richter, N. L. D. Somasiri, "Conducting Polymers", Luis Alcacer (ed.), Reidel Publ, Dordrecht, Holland, 1987, 105; M. Angelopoulos et. al., Mol. Cryst. Liq. Cryst., Id.; S. Wang, F. Wang, X. Ge, Synth. Met., 16 (1986) 99; J. P. Travers, J. Chroboczek, F. Devreux, F. Genoud, M. Nechstein, A. Syed, B. M. Genies, C. Tsintavis, Mol. Cryst. Liq. Crystl., 121, (1985) 195-100.